Many systems require oscillators, particularly those having two or more modules communicating with each other where an oscillator is often present in each module. For reliable serial communications, it is desirable that oscillators in communicating modules have good stability and operate reasonably close to a desired nominal operating frequency—for this reason, many modules incorporate crystal or ceramic resonator oscillators. Crystals and ceramic resonators, however, are expensive.
In order to avoid the expense of crystals and ceramic resonators, relaxation oscillators have been used, unfortunately these typically operate at low frequencies and require precision components such as external resistors and capacitors.
It is well known in the art that standard integrated circuit processing produces resistors and capacitors having significant process-related variation coupled with high temperature and voltage coefficients, although it is possible to make resistors and capacitors having fairly precise ratios. Further, transistor threshold voltages and saturation currents also often vary. While on-chip precision devices can be fabricated using additional resistance layers and laser-trimming, extra layers and laser-trimming add significant costs to integrated circuit manufacturing. For this reason, on-chip relaxation oscillators often use one or more external precision resistors and/or capacitors to determine operating frequency.
External precision resistors and/or capacitors not only add to system cost, but require circuit board area and dedicated pins on the integrated circuit to permit connection to these external parts. Further, typical relaxation oscillator designs operate at a lower frequency than is desirable in some applications.